1. Field of the Invention
This invention relates generally to a flexible manufacturing system and more specifically to such a system utilizing modular work stations configured to provide asynchronous operations on a plurality of work pieces mounted on fixtures or pallets that are transported through the system by means of a conveyor arrangement.
2. Background Art
In the manufacturing of certain metal workpieces to provide finished metal products, it is not possible to achieve the final piece shape and tolerances by metal casting, forging, and extrusion or forming alone. Part manufacture usually includes several different machining operations that must be applied to the original casting/forging, such as shaping, drilling, boring, milling, cutting, and tapping. Such final products may be utilizable in any of a number of industries, for example, in the automobile industry, to manufacture, without limitation, such products as engine manifolds, wheels, brake rotors, water and oil pumps, oil pan, engine covers, valve covers and suspension components. While the invention is described herein for use to manufacture metal components, it will be readily understood that the features of the invention can also be used to manufacture other types of parts, for example, to provide secondary operations on plastic parts or composite materials.
In mass production, it is necessary to organize a manufacturing process with high reliability, flexible part operations, short cycle times, easy maintenance, and worker safety while minimizing cost and space requirements. It is also highly desirable to achieve flexibility of the manufacturing process and the manufacturing equipment so that the process and equipment can be adapted at low cost to changes in part design, part mix, or part quantities, and is also easily adaptable from one configuration to another depending on the precise part which is being manufactured.
Flexible manufacturing systems utilizing automation are preferred because of increased quality, consistency and reduced set-up time and overall cost, resulting from increased efficiency, process control and reduction in need for manual labor. Flexible manufacturing systems have been used, inter alia, in cutting, forming, grinding, deburring, heat treatment and other operations performed on metal workpieces to render them into a product of desired size and configuration. Exemplary components of such flexible manufacturing systems include a plurality of machining centers and, optionally, may be applicable to numerically controlled machine tools, which tools directly process the workpieces by cutting, shaping or otherwise finishing the workpieces.
Automated machining systems typically utilize conveyors to move parts from one machining center to another, usually mounted on a fixture that itself may be mounted on a conveyor pallet. The workpieces are normally delivered to and removed from the machining station automatically by a conveyance, such as a track or rail arrangement. When a part is brought to the desired machining center, the part is optionally removed from the pallet, and loaded into and unloaded from the machine tool by auxiliary material handling devices, such as, for example, pick-and-place devices, loader gantries, multi-axis robot arms or sometimes on integrated pallet change devices. Transportation of the pallets, whether empty or loaded, may be performed by means of a dedicated track structure which would carry pallets from a pallet supply to the set-up station, and then to the machining centers and the numerically controlled machine tool under the control of a system controller. A flexible manufacturing system of this type further may preferably include a chip removal station, a workpiece cleaning station, a part inspection station, a parts marker and tool stacker, all of which are associated with other components of known systems.
The workpieces are fixed to pallets in a set-up station and then transferred to each of the machining centers and the numerically controlled machine tool for further operational steps. However, in most instances, the parts have to be removed from the pallets and transferred onto another fixture, associated with the machining station, which will receive a workpiece (e.g., an unfinished casting) mounted and automatically perform the desired machining operations on the workpiece. After the operations are completed at that station, typically, the parts are removed form the machining station fixture and remounted to the pallet for further transport to the next operation along the conveyor and additional machining operations.
Alternatively, in a few high end applications, parts are premounted on a fixture and transported on pallets between machining stations. In these applications, the transport of the parts to be machined are moved together with the fixtures (pallets) from the conveyor to an adapter associated with the specific machining center, as will be described below.
Various types of machining stations are known. Dedicated stations are constructed to perform a fixed set of operations and cannot be easily adapted to perform other tasks. Dedicated stations usually have a cost advantage when a large volume of parts are to be made and no significant design changes are necessary during a long production run. Another type of station uses computer-numerically-controlled software. Machines of this type are typically referred to as CNC machines. These are programmable to perform a variety of machining operations and are capable of producing parts with a lower number of machining stations and therefore requiring less relocation of a workpiece during manufacture. In addition, CNC machines are more easily adapted to new products or processes and can reduce overall capital investment for a changeover by decreasing the changeover time necessary between operations. A typical CNC machine has programmable multidimensional movement of both the tool head and the table that receives the fixture and workpiece. In most typical applications, CNC machines require a part-clamping fixture, designed and constructed to accept the specific part being processed, be attached to the table of the machine. These fixtures must be removed and replaced in the machine station with each part change. These types of fixtures, and the pallet pools required to be kept in stock, also tend to be relatively expensive.
These types of conventional flexible manufacturing systems, as well as other types, normally employ a single type of machining center, i.e., either vertical machining centers or horizontal machining centers. Since the vertical machining centers cannot normally work on a side surface of a workpiece, and likewise a horizontal machining center cannot normally work on a top surface of a workpiece, a need exists to turn the workpiece through any number of angles up to and including 90 degrees from the initial insert angle into the machining station of a workpiece to expose the workpiece surface requiring machining toward the specified tools having a specific orientation at that workstation. The machining centers capable of providing manipulations of a workpiece in almost all orientations, sometimes referred to as the degrees of freedom of motion, are rare, and thus a part may be required to be operated on at two separate work stations to complete the manufacturing thereof, with the possible requirement of changeover in the fixture between the two operations. This usually requires cumbersome and time-consuming manual workpiece set-up operations, which utilize labor-intensive processes. For example, if a workpiece is required to be processed by both a horizontal and a vertical machining station, it may become necessary to change the pallet or base on which the workpiece is mounted, or conversely, to transfer a pallet that is used for horizontal machining onto an adapter pallet that can provide for vertical machining, as is disclosed by U.S. Pat. No. 5,927,463 to Lee, so as to have compatibility with the specified machining center.
In view of the foregoing, it is desirable and even necessary to provide for an arrangement that uses a single pallet providing the fixture for both horizontal and vertical machining, in combination with a manipulation tool that is capable of improving the machining efficiency of work pieces while simultaneously providing a multi-axis positioning capability to the workpiece. As used herein, the term “horizontal pallet” is intended to mean a pallet particularly adapted for use with a horizontal machining center and the term “vertical pallet” is intended to mean a pallet particularly adapted for use with a vertical machining center.
To provide some flexibility in system configuration, combined vertical and horizontal machining centers may be used in a single flexible manufacturing system despite the possible incompatibility of their pallets. For example, U.S. Pat. No. 6,745,454 to Grimshaw et al. describes an overhead gantry system to transfer the workpieces between machining stations. Other known systems, for example, those described in aforementioned U.S. Pat. No. 5,927,463, describe workpiece stations that have both a pallet utilizable in a vertical machining center and an adapter that operates as a horizontal pallet so that a transfer of a vertical pallet to the horizontal adapter is necessary, depending on the desired machining operation. This transfer to the adapter necessarily requires the horizontal pallet to be fixed, e.g., clamped, onto the vertical pallet adapter and then relocated and indexed in relation to the workpiece prior to being fed to the vertical machining center, and vice versa. Thus, full automation of the flexible manufacturing system is not achieved, and the multiple clamps that are required between the workpiece and the machining station often lead to loss of precision and of repeatability in positioning, leading to a loss of precision and in repeatability thereby greatly reducing the productivity or yield rate and making the manufacturing process less economical.
An alternative adaptable type of CNC machine may be provided by the addition of a tool changer and tool magazine to the basic machine tool. In this configuration, the machine tool has a single spindle shaft, with a tool holder mounted to the distal end that can automatically grip and release a universal tool mounting feature. Also included in this machine configuration is a tool changer mechanism that can insert and remove tools from the tool mount. Further included in this machine configuration is a magazine device which can hold, and selectively present to the tool changer mechanism, a plurality of tools, each of which is prepared for a specific operation within the machining sequence designated for the manufacture of a part. At appropriate times during the execution of a machining program sequence, the CNC controller will call for the next required tool. The machine spindle will stop, move to the required location, and the tool magazine will present the required tool to the tool changer. The tool changer will then retrieve the required tool from the magazine, remove the current tool from the machine spindle, and replace it with the new tool. Once the tools have been exchanged in the spindle, the machine tool returns to its machining sequence, and the tool changer places the “old” tool into its appropriate location within the tool magazine for future retrieval, when needed.
The above-described systems have provided increased automation in the prior art devices. Nevertheless, previous automated systems have suffered from various drawbacks. For example, transfer of workpieces between work stations has remained labor intensive, slow, and/or inflexible (i.e., not easily adaptable to process changes or substitutions). A single workpiece may need to be swapped between various fixtures, or pallets, that are associated with and correspond to different machining stations when the particular set of machining operations to be performed on the workpiece occurs at several different machining stations. Overall precision, accuracy and repeatability of positioning suffers due to a loss of an exact registration in a reference position between fixtures or the requirement for multi-axis positioning and orientation. Another disadvantage has been the inaccessibility of the CNC machines during operation, making observation and maintenance more difficult. It is logical that reducing the number of adapters fixing the workpiece to the machining station workbase necessarily will reduce the possible error that may arise from inaccurate registrations between the various levels of adapters disposed between the workpiece, pallet and the workbase.
None of the prior art systems or methods known heretofore teach a flexible system capable of interoperability without requiring the removal and transfer of a workpiece or workpiece/pallet combination from one pallet capable of processing the workpiece in one orientation to another pallet capable of processing the workpiece in another orientation, or conversely, requiring vertical withdrawal of the pallet and workpiece structure, for example, by a gantry system. Heretofore known pallet transporting systems require the synchronous processing of separate workpieces at plural work stations, after which the pallets are sequenced simultaneously to the next work station where the immediately next operation(s) are performed on the workpiece. However, such systems, commonly called “transfer lines,” are suited only for large batch jobs where the same parts or family of parts are made in great numbers. It is not well suited to providing smaller numbers (on the order of 1-3 parts, for example) of finished workpieces or different parts, for example ones or twos of a particular part that are made on a monthly basis. Also, it cannot provide easy interchangeability of the operations provided by the plural workpiece machining centers.
These and other disadvantages are overcome by the present invention.